Update version to 0.3.0 in preparation to Composer package release

[gziolo]

Similar process as with #90.

This PR prepares the branch to release v0.3.0 based on the work accomplished in pre WP 6.9 milestone:

• Replaces @since n.e.x.t with @since 0.3.0
• Bumps version from 0.2.0 to 0.3.0
• Includes coding style fixes necessary to pass CI check in WordPress core.

The same version was tested against WP core with WordPress/wordpress-develop#9410 and all CI checks passed with the changes applied.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.65%. Comparing base (9ffcc0e) to head (5bb3a3e).
⚠️ Report is 1 commits behind head on trunk.

Files with missing lines	Patch %	Lines
includes/bootstrap.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #116   +/-   ##
=========================================
  Coverage     86.65%   86.65%           
  Complexity      148      148           
=========================================
  Files            18       18           
  Lines           982      982           
  Branches         92       92           
=========================================
  Hits            851      851           
  Misses          131      131           

Flag	Coverage Δ
javascript	93.04% <ø> (ø)
unit	84.70% <88.88%> (ø)

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[github-actions]

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Co-authored-by: gziolo <gziolo@git.wordpress.org>
Co-authored-by: emdashcodes <emdashcodes@git.wordpress.org>

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[emdashcodes]

Looks good. Confirmed the PR only contains version updates and a few minor spacing fixes.